1. Field of the Invention
The present invention relates to an anchor bolt having a substantially cylindrical stem and at least one thread formed, preferably, as one-piece with the stem and extending from the front free end of the stem over at least a portion of the longitudinal extent of the stem.
2. Description of the Prior Art
For attachment of constructional parts to a constructional component, in particular, to concrete or brickwork, in addition to plastic and metal dowels, also anchor bolts are used. Preferably, anchor bolts are used in through-mounting and for temporary attachment when an easy and rapid dismantling capability is needed. The anchor bolts are formlockingly anchored because, during the setting process, they form a thread in the wall of the preliminary formed bore. To this end, the anchor bolt stem is provided with a thread which, upon rotation of the anchor bolt, cuts and mills the wall surface. The properties of known anchor bolts, in particular, of anchor bolts used for attachment of constructional parts to a concrete, are defined by the diameter of the bore in which the anchor bolt to be anchored, i.e., by the diameter of the anchor bolt system, by flank height of the thread, its steepness, and setting depth. The obtainable holding forces depend on the setting depth, stem diameter, the quality of the constructional component, e.g., of concrete, etc.
The adjustment of attachments effected with anchor bolt is relatively easy and can be done easily and quickly. The known anchor bolt must, however, be set-in rather deeply in order to obtain the necessary holding forces. In comparison with an undercut system, they use the strength of the concrete only to a small degree. At small setting depths, the conventional anchor bolts, in particular under dynamic loads, have often unsatisfactory load resistance values. With a tensile load, there is a danger of formation, in the surface of the constructional component, of conical outbreaks which occur as result of introduction of force peaks in the surface region. As a result, the constructional component shears off, as a zipper, along the turns of the outer thread until a total breakdown of the anchor bolt attachment occurs.
Accordingly, an object of the present invention is to eliminate the drawbacks of the prior art anchor bolt and to provide an anchor bolt, in particular a threaded anchor bolt embedded in concrete, having, in comparison with the conventional anchor bolts, favorable load resistance characteristics.
Another object of the present invention is to provide an anchor bolt with which necessary load resistance values are obtained, in particular under a dynamic load even when it is set at a small depth.
These and other objects of the present invention, which will become apparent hereinafter, are a achieved by providing an anchor bolt having a substantially cylindrical stem and at least one thread formed thereon, preferably, as one-piece part, and extending from a free front end of the stem at least over a portion of the stem longitudinal extent. The thread turns have at least over a front one/third of the stem, an axial flank width which decreases, with an increase, of the distance of respective thread turns from the free front end of the stem, so that the distance between two following each other thread flanks of two following each other thread turns of the thread increases backwards, i.e., in a direction opposite to the setting direction of the anchor bolt.
The shape of the thread according to the present invention permitted to obtain with the anchor bolt load characteristics comparable with those characterizing an undercut system. As a result of the thread shape, at a tensible load acting on the anchor bolt, the load is introduced in the bore deepest region. This provides for much better use of the concrete strength. The thread turns in the surface region do not apply any shearing forces to the thread cut in the bore wall upon application of the tensible load. As a result, the formation of cone outbreaks and a zipper shearing effect are prevented.
According to an advantageous embodiment of the present invention, the trailing flanks of the thread turns are spaced from respective reference planes, which extend transverse to a stem axis and pass through transitional points from leading flanks to respective regions of the thread turns spaced radially from the stem by a largest distance, by an axial distance, by an axial distance that decreases with an increase of a distance the respective thread turns are spaced from the free front end of the stem. the thread turns, which are located in the region of the free front end of the stem, have a larger axial flank width than the following backwards thread turns. Therefore, it is the front thread turns of the anchor bolt that define the width of the thread formed in the concrete. Therefore, the frontmost thread turns are almost completely embedded in the thread ridging formed by the frontmost thread turns of the anchor bolt in concrete. The axial width of the following thread turns decreases so that the leadings flanks still abut the limiting surfaces, adjacent to the bore deepest region, of the thread ridging of the thread formed in the bore wall. The following thread turn flanks, with an increases of their distance from the free front end of the stem, become spaced from the limiting surfaces of the cut thread ridging by ever increasing distances. As a result, it is insured that upon application of a tensile load, no shear stresses are generated in the surface region of the concrete.
The reduction of material on the thread can be achieved by forming the thread so that the trailing thread flanks form with a reference plane, which extends transverse to the thread turn, an angle which decreases in the backward direction. For manufacturing reasons, it is, however, preferred when the trailing flanks of two adjacent threads turns extend parallel to each other or substantially parallel.
In order to insure the introduction of load forces into the deepest region of the bore, even under unfavorable conditions, e.g., at a partial breakdown of the concrete, the thread has a pitch measured between the trailing flanks of two adjacent thread turns and which decreases with an increase of the distance from the free front-end of the stem. The pitch decreases linearly from the free front end of the stem backwards and passes into a thread region with a constant pitch. The thread region with a constant pitch occupy from about one/third to about three/fourth of the total thread length.
The thread has a length that occupies from about one/third to about three/fourth of the longitudinal extent of the stem. These constructive measures permit to reliably prevent shears stresses in the surface region of the constructional component.
The shape of the thread is so selected according to the present invention that the pitch measured at the trailing flank changes. However, the mean thread pitch, which is measured at references planes, extending transverse to the stem axis and passing through thread turns, remains constant.
In accordance with an advantageous embodiment of the present invention, the thread can have, in the front region of the stem, two-three thread turns having a larger mean pitch than in the remaining section of the thread. This insures a smaller deformation in the constructional component at the formlocking anchoring of the anchor bolt and an increased restoring torque. This reduces the danger of automatic loosening of the anchor bolt.
The novel features of the present invention, which are considered as characteristic for the invention, are set forth in the appended claims. The invention itself, however, both as to its construction and its mode of operation, together with additional advantages and objects thereof, will be best understood from the following reference to the accompanying drawings.